The use of equalization for audio recordings has been common practice for many years. Historically, equalization has been applied symmetrically prior to transmission and after reception, and is used to condition the signal based on the characteristics of the transmission media. Standardized calibration is often provided for playback of a given media type (e.g., vinyl records and magnetic tape media). The specified calibration may indicate fixed processing to be applied (e.g., frequency dependent gain), or a target response to be achieved (e.g., net frequency response of entire playback system).
Different transmission or storage media impose different equalization requirements. For example, on vinyl records, low frequencies are generally difficult to recover and inefficient to transmit and high frequency noise can be introduced during playback. For audio distribution on vinyl phonograph records, an industry standard RIAA (Recording Industry Association of America) playback equalization scheme was established in the mid-1950s to standardize the hundreds of different equalization schemes that the individual record companies were using. FIG. 1A illustrates a plot of the RIAA playback equalization scheme as generally known in the art. To mitigate the possible low frequency attenuation and high frequency noise of vinyl records, equalization (EQ) is applied prior to transfer to vinyl (pre-emphasis curve 102) and the symmetric/opposite EQ is applied after recover of the audio signal from the vinyl record (de-emphasis curve 104). This EQ is standardized for all modern vinyl recording and reproduction devices, and the specified frequency dependent gains are applied universally by the recording and playback devices.
Similar to vinyl records, reproduction issues arise with recording to audio cassette tapes. Again, equalization (EQ) is applied prior to transfer to tape (pre-emphasis) and the symmetric/opposite EQ is applied after recover of the audio signal from the tape (de-emphasis). For cassette tapes, the optimal EQ depends on the chemical formulation of the tape, typically Fe—Cr, Cr02, metal, or normal (Fe2O3). As a result, several standards for EQ exist and are engaged based on the type of tape used, such as through a mechanical tab on the tape that actuates a switch on the playback device.
With regard to cinema applications, the need for standardized frequency response for movie audio playback was recognized from the early days of sound-on-film. In the case of cinema, the need for EQ (non-flat frequency response) is a function many parameters including of the distribution format, the playback equipment, and the playback environment. The first attempt to codify the system was made by the Motion Picture Research Council (reporting to the Academy of Motion Picture Arts and Sciences) in 1937, and was based on the subjective preference for playback of typical cinema content played over typical equipment. The preferred response included significant high frequency attenuation. The “Academy Curve” was implemented as electrical filters within the “A-chain,” which is the part of the audio signal path that includes transduction from media to electrical signal, and any other processing prior to audio amplifiers, of the playback equipment. Later, as more advanced and varied playback equipment was employed (especially loudspeakers), the variation in the playback response increased. In response, the ISO (International Standards Organization) attempted to specify average theater B-chain response, which is the net response due to power amplifier, loudspeaker, screen, and auditorium acoustics. This curve was intended as a reference point for mix stage design to match the sound on the mix stage with what was being heard by the audience.
More recently (over 20 years ago), technical advances allowed for a flat A-chain. As a result, there was an opportunity and necessity to redefine the target B-chain response with a flat A-chain in mind and this led to the SMPTE ST 202 standard, widely known as the X-curve. The X-curve is now the standard for playback of optical and digital cinema sound tracks, and is illustrated in a representative curve of FIG. 1B. Many cinema processors provide a “format” selector, similar to the media-type selector on cassette decks, that allows the user to select the appropriate response for playback of current films (with X-curve mastering) or older movies that were mastered with the Academy curve in circuit. It should be noted that the X-Curve is a system target response curve, which takes into account the effects of the entire playback system. Frequency dependent gain (implemented as audio filters) is applied as necessary to achieve the desired target response. This is in contrast to the equalization curves for vinyl records or cassette tapes, which specify the fixed, frequency-dependent gain that is to be applied.
In some cases, a mixing studio audio engineer, or content creator may have personal preferences for the overall frequency response of the content creation or validation environment. As stated above, standardized calibration methods for playback based on a given media type (e.g., vinyl, tape, optical film etc.) generally specify a fixed processing to be applied (e.g., frequency dependent gain), or a target response to be achieved (e.g., net frequency response of entire playback system). They do not provide a method of customizing program reproduction. Though present equalization standards may provide convenience in providing a guide that is generally acceptable, these industry standard curves generally impose a response that is not necessarily represent what the content creator heard during the creation of the program. Moreover, with respect to cinema, present equalization curves were developed for film-based use where the audio is encoded in film reels that are distributed to individual cinemas. AV content (movies, TV, streaming music, etc.) is increasingly being distributed as digital data that is transmitted over physical or wireless networks. For such transmission applications there is no standard equalization curve. Thus, audio programs transmitted for playback in non-cinema environments, and especially cinema content conforming the standard X-curve, is often not well matched to the program intended to be heard by the that was heard and approved by the content creator.
It is therefore advantageous to provide a method for matching the system response of the playback environment to that of the program creation (or validation) environment and that provides a high fidelity audio presentation that is consistent from mix stage to any playback environment.
The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions. Dolby, Atmos, Dolby Digital Plus, Dolby TrueHD, DD+, and Dolby Pulse are trademarks of Dolby Laboratories.